With increases in the amount of data communication in recent years, the need has been intensified for a mobile communication system having higher spectral efficiency, and various techniques have been proposed with the aim of actualizing the system. OFDMA (Orthogonal Frequency Division Multiple Access) is one of techniques having the possibility of enhancing the spectral frequency, and is determined to be applied to the downlink access scheme of the E-UTRA (Evolved Universal Terrestrial Radio Access) system of which standardization has been proceeding especially in 3GPP (The 3rd Generation partnership Project) (Non-patent Document 1).
This OFDMA system is a system in which users in a cell access to respective resource blocks divided by time and frequency, and is capable of performing scheduling for assigning users to resource blocks providing good quality, and controlling transmission parameters such as a modulation scheme, coding rate, transmit power and the like for each resource block. Herein, in order to perform scheduling of users and controlling the modulation scheme/coding rate for each resource block suitably, it is necessary for the transmitting side to grasp channel conditions on the receiving side. Therefore, the need arises for the receiving side to notify (as feedback) the transmitting side of reception conditions. Such feedback information about channel conditions is called CQI (Channel Quality Indicator) in the E-UTRA system.
As described above, the receiving side needs to send back the CQI to the transmitting side in adaptive control of a modulation scheme and the like, and when a lot of CQIs are sent back, the problem arises that the spectral efficiency on uplink severely degrades. One of means for solving the problem is a CQI compression method using discrete cosine transform (hereinafter abbreviated as “DCT”) (Non-patent Document 2).
Shown herein is an example in the case of performing DCT processing on reception quality information (reception quality measurement result). FIG. 1 is a diagram showing an example of the reception quality information, and FIG. 2 is a diagram showing an example of a result of performing DCT processing on the reception quality information as shown in FIG. 2. FIG. 1 shows the reception quality information (CQI (Received SNR)) in association with the subcarrier number (Sub-carrier Number). Further, FIG. 2 shows an absolute value (Absolute Value after DCT) of a sample value subjected to DCT processing indicative of a result (signal component) of performing DCT processing on the reception quality information in association with the sample number (Sample Number).
As shown in FIG. 1, in the case of performing DCT processing (the number of points is “1024”) on the CQI (the number of subcarriers is “1024”) varying continuously in the frequency region, the result of the DCT processing is indicated as shown in FIG. 2 as an example. The DCT-processed signal components gather in the low-frequency region as shown in FIG. 2, and high-frequency components are an extremely small value (nearly zero). By exploiting such a property, Non-patent Document 2 provides the method of sending back only the low-frequency components without sending back the high-frequency components of the signal subjected to DCT, and thereby compressing the feedback amount of CQI. FIG. 26 is a diagram showing an example of a state where the low-frequency components are only sent back. After receiving CQI which is compressed in this way, the transmitting side inserts zero in sample points of deleted high-frequency components, performs inverse discrete cosine transform (hereafter abbreviated as “IDCT”), and is able to reproduce the CQI observed on the receiving side, while hardly undergoing the effect of deleted high-frequency components.
Non-patent Document 1: 3GPP, TR 25.814 v0.3.1, “Physical Layer Aspects for Evolved UTRA”
Non-patent Document 2: 3GPP, TSG RAN WG1 ad hoc meeting on LTE, R1-060228, “Sensitive of DL/UL Performance to CQI Compression with Text Proposal”